The invention relates to a piston-cylinder assembly as defined in the introductory part of claim 1. Such a piston-cylinder assembly was introduced at the 1973 MAHLE Colloquium in the lecture on xe2x80x9cPendulum Shaft Pistonxe2x80x9d. Three shaftless pistons with diameters of 111.1 mm, 82 mm and 75 mm, which had been tested in an engine test, were introduced in picture 2 of the lecture. It was stated in the associated report on the test results that the ring zone of such pistons is quite capable of guiding itself, and that substantially reduced ring zone clearances can be achieved in this way. It was subsequently stated that a shaftless piston was of course not considered for the series in the light of existing operating conditions under which lateral guidance of the connecting rod was not adequately assured.
Another drawback of such pistons is high wear of the supporting surfaces, in particular edge wear.
Furthermore, the narrow clearances feasible and required with such a design are advantageous only as long as no flaws occur in the combustion process, which is controlled by the characteristic field. Piston seizing has to be expected in the event of flaws in the combustion process and an increased temperature level on the piston head.
Therefore the invention deals with the problem to design a shaftless piston ready for series production and to avoid the drawbacks stated above.
With the piston of the type specified above, said problem is solved by the characterizing features of claim 1. The objects of the dependent claims are advantageous further developments.
Guiding the piston by one or, if need be, two supporting rings fitted on the bosses, whereby the minimum height of such rings required for assuring adequate stability can be determined in engine tests, and whereby such minimum height is primarily dependent upon the weight and the diameter of the piston and on its speed, assures sufficient lateral guidance of the connecting rod and distinctly reduces edge wear. By guiding the piston both in the zone of the ring part and in the zone of the supporting ring, provision can be made in the zone of the ring part of the piston for clearances that will not yet lead to seizing of the piston even under unfavorable or unscheduled operating conditions.
As compared to pistons with shafts, and in particular to pistons with self-aligning shafts, omission of the shaft results in a distinct weight advantage as well as price advantage. However, said advantages have to be paid for by the drawback that the advantages achieved with the articulated design are cancelled.
Supporting rings on pistons are admittedly known per se, for example from U.S. Pat. No. 2,966,382; however, only in connection with pistons which also have a shaft. Such supporting rings are normally made of an iron or steel material and are expected to protect the shaftxe2x80x94which consists of an Al-materialxe2x80x94against abrasive wear. Not known are supporting rings which are directly fitted on the bosses.
The invention is basically based on a top part of a pendulum shaft piston, where instead of a shaft that is connected with the piston in an articulated manner, a simple supporting ring is fitted on the bosses preferably at the level or in the zone of the axis of the pin.
If required, a second supporting ring can be fitted on the bosses for further stabilization.
The supporting ring may also be constructed in two parts.
If needed, the supporting ring may also be coated with gliding layers customary in pistons.
With pistons having a cooling channel in the piston head, and where the lower limiting wall is formed by a two-part circularly shaped metal sheet resting on its inner radius in a groove in the boss support, fixing of the metal sheet on the outer radius can be advantageously accomplished with a spacer sleeve clamped between the supporting ring and the metal sheet, such spacer sleeve having an inside diameter approximately corresponding with the inside diameter of the supporting ring. The required clamping of the spacer sleeve can be made possible by a design of the supporting ring according to claim 2.
If the supporting ring is fixed approximately at the level of the axis of the pin, the supporting ring may also assume the function of the pin safety rings by means of mushroom safety elements made of plastic, which are known in the state of the art, such elements being pushed into the pin on the face side and capable of being moved against the supporting ring.
When a single-part slotted supporting ring is employed, the mushroom safety element may be designed also as a non-rotary safety element for the supporting ring by providing the mushroom element with two grooves separated by a bridge, with each of said grooves receiving a butt end of the supporting ring, and with the bridge being disposed in the joint of the ring. With adequately narrow axial fitting of the ends of the supporting ring in the grooves, such fixation at the same time stabilizes the running behavior of the supporting ring within the area of the joint of the ring.
The piston-connecting rod assembly is mounted by pushing the supporting ring over the small eye of the connecting rod when the latter is ready for installation. The connecting rod is subsequently connected with the piston by the pin. Thereafter, the supporting ring is fixed on the bosses, or, with inwardly clamping supporting rings, inserted in a groove produced in the bosses by turning.
All commonly used piston materials can be considered for the piston and the supporting ring. The supporting ring is preferably produced from GGL or GGG.
The supporting rings may have a rectangular or trapezoidal cross section, or a U-profile or T-profile.
The supporting ring preferably rests directly on the bosses without interconnecting adapter pieces, or in a groove turned into the bosses. However, adapter pieces can be used, if need be.
The invention is explained in the following on an exemplified embodiment. In the drawing,